Roll back aggressive deletion of cache. Fixes #1091.

* sbt 0.12.3 introduced 482f9905235217c339934d8a7a560aae13b7f296 (aggressive deletion of cache) to fix #637 and #641.
* The underlying issue of #641 was an incorrect classifier, and it was
  fixed in 718fa9177254e6067cafdc1ee6a744ba5c325921 for #683 and shipped as sbt 0.13.0.

We should roll back 482f9905235217c339934d8a7a560aae13b7f296 to fix #1091 (re-download of SNAPSHOTs). Rolling it back should not cause regression of #641 as confirmed by the scripted test `scripted dependency-management/cache-classifiers` and its externalized version [eed3si9n/test-sbt-641][1].

  [1]: https://github.com/eed3si9n/test-sbt-641